1. Field of the Invention
The present invention relates to a diffractive optical element and also to an optical system incorporating such a diffractive optical element. More particularly, the present invention pertains to a diffractive optical element, as well as to an optical system, suitable for use in optical devices and apparatuses such as, for example, photographic cameras, video cameras, binoculars, and projection exposure apparatuses used in semiconductor production.
2. Description of the Related Art
Various types of optical systems have recently been proposed, among which are noted those which incorporate diffractive optical elements capable of diffracting light rays, such as Fresnel zone plates, diffraction gratings, holographic elements, and so forth.
In general, a diffractive optical element is used as an optical element that converts an incident wavefront into a predetermined wavefront. Diffractive optical elements exhibit characteristics that cannot be obtained with ordinary refractive lenses. For instance, diffractive optical elements show dispersive power which is reciprocal to that of refractive lenses. In addition, diffractive optical elements do not have substantial thickness and, therefore, serve to reduce the size of an optical system when incorporated in such an optical system.
Techniques employed in semiconductor production can be applied to production of diffractive optical elements, provided that the elements are designed to have binary configurations. This permits rather easy production of diffraction gratings even when the gratings have very small pitches. For this reason, intense studies are being made on binary-type diffractive optical elements in which blazed configurations are approximated by step forms.
FIGS. 9A to 9C are illustrations of a concept of a binary-type diffractive optical element. More specifically, FIG. 9A shows a plano-convex refractive lens 701 and FIG. 9B shows a kinoform-type diffractive optical element, i.e., a Fresnel lens, denoted by 702. The element 702 is formed by removing, from the lens 701 shown in FIG. 9A, portions which provide optical path differences amounting to integer multiples of wavelength. FIG. 9C shows a binary-type diffractive optical element which is obtained by binarily approximating the configuration of the Fresnel lens 702: namely, by quantizing the configuration of the Fresnel lens 702 into step form with a step height which is a fraction of the wavelength. Numerals 703 and 705 appearing in FIGS. 9B and 9C denote a transparent substrate. In FIG. 9B, the surface of the transparent substrate has been processed to have a minute pattern which provides the kinoform-type diffractive optical element, whereas, in FIG. 9C, the surface of the substrate 705 has been processed to have a minute step pattern which implements the diffractive optical element 704. As stated before, the approximation of the diffraction grating pattern by the step configuration permits the introduction of ordinary LSI production techniques into the production of the diffractive optical elements.
FIG. 10 shows, by way of example, a conventional process for producing a quadruple or 4-level binary-type diffractive optical element. There are shown a transparent glass substrate 800 having a refractive index xe2x80x9cnxe2x80x9d, a resist 801, and a mask 802 used for a primary exposure. Numeral 803 designates exposure light rays. By way of example, a positive resist is used as the resist 801.
In Step A, the pattern of the mask 802 is transferred to the resist 801 by exposure of the resist 801 to the exposure light 803 through the mask 802. In Step B, the resist is subjected to development and, in Step C, etching is effected on the glass substrate 800. Removal of the resist from the substrate 800 is performed in Step D, whereby a duplex or 2-level binary-type diffractive optical element is obtained.
The penetration or depth d1 of etching is determined to meet the condition of:
d1 =xcex/2/(nxe2x88x921)
where, xcex represents the wavelength of light to be diffracted by the binary-type diffractive optical element.
The glass substrate 800 now carrying the duplex binary-type diffractive optical element formed thereon is then coated with a resist 804. A secondary exposure is performed through a mask 805 in Step E. The pattern of the mask 805 has a pitch that is half the pitch of the pattern of the mask 802. The secondary exposure is conducted such that the edges of the light-shielding regions of the mask 805 are correctly aligned with the edges of the duplex binary-type configuration, so that a pattern as illustrated can be obtained through a developing processing conducted in Step F.
A secondary etching is executed in Step G, followed by Step H which removes the resist, whereby the aimed quadruple, i.e., 4-level, binary-type diffractive optical element is obtained. The depth xe2x80x9cd2xe2x80x9d of etching is determined to meet the following condition:
d2=xcex/4/(nxe2x88x921)
Although a quadruple structure has been specifically mentioned in the foregoing description, it will be obvious that an octet, i.e., 8-level, and hexadecimal, i.e., 16-level, binary structures are obtainable through repetition of the described process.
The described process basically provides binary structures having 2n steps, where xe2x80x9cnxe2x80x9d represents a natural number. However, any desired number of steps can be obtained when the number of masks employed and the widths of pattern lines are suitably selected.
It is true that the diffraction efficiency is reduced to a certain extent as a result of approximation by the stepped configuration. The diffraction efficiency, however, is still as high as about 95% in the case of an octet step structure (8-level) and about 99% in the case of a hexadecimal (16-level) structure, thus ensuring a high level of practical utility.
In most cases, plane parallel plates are used as substrates of diffractive optical elements, because of ease of manufacture. For instance, Japanese Patent Laid-Open No. 7-128590 proposes a diffractive optical element formed on a planar substrate. On the other hand, it is a common understanding that a greater degree of versatility is obtained in regard to correction of aberration when a flat surface of the substrate is substituted by a spherical or an aspherical surface.
Japanese Patent Laid-Open No. 62-229203, as well-as Japanese Patent Laid-Open No. 6-242373, discloses a diffractive optical element formed on a substrate having a spherical or aspherical surface.
Use of a diffractive optical element as a component of an optical system offers various advantages such as ease of correction of aberration, improved transmittance and so forth, depending on the manner of use. However, fabrication of a binary-type diffractive optical element, relying upon repetition of a lithographic process as described before, encounters restrictions due to practical limitations in the exposure process in regard to precision of the mask alignment, minimum pattern line width, and so forth. The performance of a diffractive optical element is ruled mainly by the minimal pitch of the diffraction grating. It has been impossible to produce a binary-type diffractive optical element having a minimum grating pitch of less than 2 or 3 xcexcm.
A diffractive optical element of the type shown in FIG. 9C simulating the function of a convex lens, as well as diffractive optical elements simulating the functions of a concave lens, convex mirror and concave mirror, usually have a diffraction grating the pitch of which progressively decreases from the center towards the peripheral end of the element. A greater refractive power of the diffractive optical element requires a smaller pitch of the grating and, hence, a correspondingly smaller width of the minimum pattern line. Consequently, the exposure process in the production of a diffractive optical element has to sustain more strict conditions in regard to the precision of mask alignment and fineness of the pattern line.
Accordingly, it is an object of the present invention to provide a diffractive optical element which is easy to manufacture and also an optical system incorporating such a diffractive optical element.
In accordance with one aspect of the present invention, a diffractive optical element comprises a substrate and a diffraction grating formed on the substrate. The pitch of the diffraction grating varies such that it progressively decreases radially outward over a radial region from the center and is then kept substantially constant towards the radial extremity of the element.
An optical system in accordance with this aspect incorporates a diffractive optical element having the features set forth above.
According to another aspect of the present invention, a diffractive optical element has a substrate, and a diffraction grating and an aspherical surface formed on the substrate. The pitch of the diffraction grating varies such that it progressively decreases radially outward over a radial region from the center and is then kept substantially constant over a predetermined radially outer portion towards the radial extremity of the element. The portion of the aspherical surface corresponding to the above-mentioned radially outer portion over which the grating pitch is substantially constant is a curved surface. An optical system in accordance with this aspect of the invention employs a diffractive optical element having the described features.
In accordance with still another aspect of the present invention, an optical system comprises: a first substrate having a diffraction grating the pitch of which progressively decreases radially outward over a radial region from the center and is then kept constant over a radially outer portion of the grating; and at least one second substrate having an aspherical surface, wherein the portion of the aspherical surface corresponding to the portion of the first substrate over which the grating pitch is substantially constant is a curved surface.
Preferably, the diffraction grating in each aspect is a kinoform-type grating or a binary-type grating.
The present invention also provides an exposure apparatus in which an exposure light beam is focused on a substrate through any of the optical systems stated above.
The invention further provides a process for producing a device having the steps of transferring a device pattern onto a resist on a substrate through the exposure apparatus, and subjecting the substrate having the resist carrying the transferred device pattern to a developing treatment.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments when the same is read in conjunction with the accompanying drawings.